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Re: [Xen-devel] question about running vm change its mem maxsize

Thank you for your reply

Daniel Stodden åé:
On Mon, 2007-03-19 at 09:20 +0800, tgh wrote:
I read the code of xc_linux_build() and xc_domain_setmaxmem(),and I am confused about how does "xm mem-max" change the max size of mem for a running VM

In xc_domain_setmaxmem() ,the XEN_DOMCTL_max_mem has been called ,and just does "d->max_pages = new_max;"

that variable determines the maximum size. the code verifies that the
new size won't be below the previous one, and therefore just needs to
readjust it.

it doesn't actually have to allocate memory. this is done on demand,
i.e. as soon as the domain references a new page frame within it's
virtual machine address space.

I see
while in the xc_linux_build(), before the vm boots up,the maxsize pfn is alloced in an array with fixsize, page_array = malloc(nr_pages * sizeof(unsigned long))

i suppose you misunderstood what that call really does. it's not
changing the maximum vm size, but allocating the initial number of pages
required to load the guest operating system image into. that's typically
much less than d->max_pages.

"page_array = malloc(nr_pages *sizeof(unsigned long))" in xc_linux_build() is 
not to allocate the physical memory to the VM,then which code or function allocate the 
phy-mem to the VM?

I am  confused about it
then how does a running vm changes its mem maxsize ,especially for expanding its mem maxsize?

for an unprivileged guest os, there is no such call. similar in the way
your desktop machine (hopefully) won't call dell ordering more RAM
without your knowledge.

changing machine size is an administrative operation. it is defined
during VM creating, and potential subject to refinement by the
administrator on dom0.
I see it is domain0 who has the control interface to set and change the max-size of the VM memory

when VM is running, we can "xm mem-set " or "xm mem-max "in the domain0'console to change the vm memory

I have used it , and want to know what happen in the xen ,espacially in a paravirtVM

could you help me
Thanks in advance

what the guest system does in order to get 'more memory' is, from the
guest system point of view, more or less equivalent to what the guest
operating system on a physical host would do: manage physical memory,
find a free page frame, map it in one or more page tables, then start
using it. xen detects that process of mapping this page frame, and
performs the allocation. but, how this is done in detail largely depends
on the type of VM (hardware-based, paravirtual, etc), and even then
there are different variants.



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